The present invention relates generally to heat-transfer labels and more particularly to a novel heat-transfer label particularly well-suited for use on untreated polyethylene surfaces and the like.
Heat-transfer labels are commonly used in the decorating and/or labelling of commercial articles, such as, and without limitation to, containers for beverages, essential oils, detergents, adverse chemicals, and health and beauty aids, as well as flat surfaces. As can readily be appreciated, heat-transfer labels are desirably resistant to abrasion and chemical effects in order to avoid a loss of label information and desirably possess good adhesion to the articles to which they are affixed.
One well-known type of heat-transfer label is described in U.S. Pat. No. 3,616,015, inventor Kingston, which issued October, 1971, and which is incorporated herein by reference. In the aforementioned patent, there is disclosed a heat-transfer label comprising a paper sheet or web, a wax release layer affixed to the paper sheet, and an ink design layer printed on the wax release layer. In the heat-transfer labelling process, the label-carrying web is subjected to heat, and the label is pressed onto an article with the ink design layer making direct contact with the article. As the paper sheet is subjected to heat, the wax layer begins to melt so that the paper sheet can be released from the ink design layer, a portion of the wax layer being transferred with the ink design layer and a portion of the wax layer remaining with the paper sheet. After transfer of the design to the article, the paper sheet is immediately removed, leaving the design firmly affixed to the article and the wax transferred therewith exposed to the environment. The wax layer is thus intended to serve two purposes: (1) to provide release of the ink design from the web upon application of heat to the web and (2) to form a protective layer over the transferred ink design. After transfer of the label to the article, the transferred wax release layer is typically subjected to a post-flaming technique which enhances the optical clarity of the wax protective layer (thereby enabling the ink design layer therebeneath to be better observed) and which enhances the protective properties of the transferred wax release.
In some heat-transfer labels, an adhesive layer (e.g., solvent-soluble polyamide, acrylic or polyester) is deposited over the ink design to facilitate adhesion of the label onto a receiving article. An example of a heat-transfer label having an adhesive layer is disclosed in U.S. Pat. No. 4,548,857, inventor Galante, which issued Oct. 22, 1985, and which is incorporated herein by reference. Additionally, in some heat-transfer labels, a protective lacquer layer is interposed between the wax release layer and the ink layer. An example of such a label is disclosed in U.S. Pat. No. 4,426,422, inventor Daniels, which issued Jan. 17, 1984, and which is incorporated herein by reference.
One phenomenon that has been noted with heat-transfer labels of the type described above containing a wax release layer is that, quite often, a degree of hazing or a "halo" is noticeable over the transferred label when the transfer is made onto clear materials. This "halo" effect, which persists despite post-flaming and which may detract from the appearance of the label, is a result of the nature of the wax coating around the outer borders of the transferred ink design layer. Hazing due to the wax release layer may also appear in "open-copy" areas of the label, i.e., areas of the label where no ink is present between the adhesive and protective lacquer layers, and also may detract from the appearance of the label.
In addition to and related to the aforementioned problem of hazing, when heat-transfer labels of the type described above are applied to dark-colored containers, the outer wax layer of the label often appears as a whitish coating on the container, which effect is undesirable in many instances. Furthermore, scratches and similar abrasions to the outer wax layer of the label can occur easily and are readily detectable.
Accordingly, to address the aforementioned issues, considerable effort has been expended in replacing or obviating the need for a wax release layer. One such wax-less, heat-transfer label is disclosed in U.S. Pat. No. 3,922,435, inventor Asnes, which issued Nov. 25, 1975. In the aforementioned patent, the layer of wax is replaced with a layer of a non-wax resin. This non-wax resinous layer is referred to in the patent as a dry release since it does not transfer to the article along with the ink design layer. In a preferred embodiment of the patent, the non-wax resinous layer comprises a thermoset polymeric resin, such as cross-linked resins selected from the group consisting of acrylic resins, polyamide resins, polyester resins, vinyl resins and epoxy resins.
Another example of a wax-less, heat-transfer label is disclosed in U.S. Pat. No. 4,935,300, inventors Parker et al., which issued Jun. 19, 1990, and which is incorporated herein by reference. In the aforementioned patent, the label, which is said to be particularly well-suited for use on high density polyethylene, polypropylene, polystyrene, polyvinylchloride and polyethylene terephthalate surfaces or containers, comprises a paper carrier web which is overcoated with a layer of polyethylene. A protective lacquer layer comprising a polyester resin and a relatively small amount of a nondrying oil is printed onto the polyethylene layer. An ink design layer comprising a resinous binder base selected from the group consisting of polyvinylchloride, acrylics, polyamides and nitrocellulose is then printed onto the protective lacquer layer. A heat-activatable adhesive layer comprising a thermoplastic polyamide adhesive is then printed onto the ink design layer.
Although the above-described wax-less, heat-transfer label substantially reduces the wax-related effects discussed previously, said label does not quite possess the same release characteristics of heat-transfer labels containing a wax release layer. Accordingly, another type of heat-transfer label differs from the heat-transfer label disclosed in U.S. Pat. No. 4,935,300, only in that a very thin layer or "skim coat" of a waxlike material is interposed between the polyethylene release layer and the protective lacquer layer to improve the release of the protective lacquer from the polyethylene-coated carrier web. The thickness of the skim coat corresponds to approximately 0.1-0.4 lbs. of the waxlike material spread onto about 3000 square feet of the polyethylene release layer.
An example of the aforementioned type of heat-transfer label, which has been used by the assignee of the present application to label polypropylene bottle caps, comprises a paper carrier web overcoated with a layer of polyethylene. A skim coat is overcoated on the polyethylene layer. A protective lacquer layer comprising vinyl and polyester resins is printed on the skim coat. An ink design layer comprising vinyl and polyester resins is printed on the protective lacquer layer. A heat-activatable adhesive layer comprising an acrylic resin, a chlorinated polyolefin and a plasticizer is printed on the ink design layer. The acrylic resin is a butyl methacrylate resin, such as ELVACITE.RTM. 2045, which is commercially available from ICI Acrylics Inc. (Wilmington, Del.). The chlorinated polyolefin is commercially available from Eastman Chemical Products, Inc. (Kingsport, Tenn.) as chlorinated polyolefin CP-343-1. The plasticizer is a glyceryl tribenzoate, such as BENZOFLEX.RTM. S-404, which is commercially available from Velsicol Chemical Corporation (Chicago, Ill.).
Heat-transfer labels of the types described above, both of the wax-release variety and of the skim-coat variety, have been used to decorate a variety of articles, including high-density polyethylene (HDPE) containers and low-density polyethylene (LDPE) containers. One example of a heat-transfer label which has been used by the assignee of the present application to decorate polyethylene (PE) containers comprises a paper carrier web overcoated with a wax release layer (approximately 6-8 lbs. wax/3000 square feet of paper carrier web). A protective lacquer layer comprising a polyester resin is printed on the skim coat. An ink design layer comprising a polyamide resin is printed on the protective lacquer layer. A heat-activatable adhesive layer comprising a polyamide resin is printed on the ink design layer.
One disadvantage associated with the use of the aforementioned label on polyethylene is that said label will not adhere to a polyethylene surface unless the polyethylene surface has previously been treated by some oxidizing technique. Typical oxidizing techniques include subjecting the polyethylene surface to corona discharge or flaming the surface with an oxidizing flame. Without wishing to be limited to any particular theory as to why pre-treatment of the polyethylene surface is necessary for the aforementioned label to adhere thereto, it is believed that untreated polyethylene is a low energy surface made up primarily of hydrocarbons whereas oxidized or treated polyethylene is a relatively higher energy surface which additionally includes ketones, carboxylic acid groups, etc. Accordingly, because the pre-treated polyethylene surface is a higher energy surface than the untreated polyethylene surface, it is more receptive to binding to the polyamide adhesive layer of the above-described label.
Another disadvantage associated with the aforementioned label is that said label, when applied to treated polyethylene, readily suffers degradation and separation from its associated polyethylene surface when the labelled polyethylene surface is "wet-flexed." "Wet-flexing" typically comprises soaking a labelled polyethylene container in a cool water bath having a sonicator for about 20 minutes, repeatedly squeezing or bending the container for a period of time, and then subjecting the container to a "tape test." A "tape test" involves applying a strip of tape to the label, removing the strip of tape from the container, and then assessing the integrity of the label. As can readily be appreciated, the inability of a label to withstand "wet-flexing" limits the usefulness of such a label on polyethylene containers of the type that are frequently squeezed under wet conditions. Because high density polyethylene containers are considerably more rigid than low density polyethylene containers and are not typically intended for applications where they will be squeezed repeatedly, the ability of a label to withstand "wet-flex" is typically less important for high density polyethylene containers than for low density polyethylene containers.
Another example of a heat-transfer label which has been used by the assignee of the present application to decorate treated polyethylene containers differs from the above-described label in that its adhesive layer comprises a different type of polyamide resin and in that its protective lacquer layer comprises a different type of polyester resin. Said label, however, also suffers from the disadvantage that it will not adhere to untreated polyethylene. Nevertheless, if a polyethylene container is pretreated under appropriate conditions, said label will adhere thereto and the resultant labelled container will pass "wet-flex."